(i) Field of the Invention:
This invention relates to long wavelength ultraviolet ray absorbers (hereinafter referred to simply as UV absorbers) and more particularly, to UV absorbers comprising dibenzoylmethane derivatives having one carboxylic acid, a carboxylate or a carboxylic acid ester group in each benzene ring thereof with or without hydrous oxides of polyvalent metallic elements.
(ii) Description of the Prior Art:
Ultraviolet rays are known to cause various dermatological effects on the skin. Dermatologically, ultraviolet rays are divided into a long wavelength ultraviolet ray having a wavelength of from 400 to 320 nm, a medium wavelength ultraviolet ray having a wavelength of from 320 to 290 nm, and a short wavelength ultraviolet ray having a wavelength not larger than 290 nm, which are, respectively, called UV-A, UV-B and UV-C.
Sunlight is a main ultraviolet ray source to which men are ordinarily exposed. Ultraviolet rays in the sunlight which arrive at the earth are UV-A and UV-B. UV-C is absorbed in the ozone layer and rarely reaches the earth. Of the UV rays which arrive at the earth, UV-B serves to form erythema and blisters when irradiated on the skin over a certain level of exposure. In addition, melanogenesis, which leads to pigmentation, is accelerated. In contrast, UV-A was believed not to cause any substantial changes in the skin. In recent years, however, it has been revealed through electron-microscopic and histological techniques that the skin suffers dermatological changes by irradiation of UV-A. In particular, the energy of UV-A arrives even at the inner skin as is different from UV-B and thus causes elastic fibers in the blood vessel walls and connective-tissue membranes to be affected slightly chronically. These changes are considered to lead to the promotion of senility of the skin. Moreover, it is also known that UV-A has the action of melanizing the skin immediately after the irradiation thereof (immediate pigmentation) and increases the degenerating action of UV-B on the skin. Thus, UV-A is considered to be one of main factors of producing or exacerbating moth-patches or freckles on the face.
As will be appreciated from the above, it is important to protect the skin not only from UV-B, but also from UV-A in order to prevent aging, moth-patches, and freckles on skin from being produced or exacerbated.
However, studies on the action of UV-A on the skin have just started and thus few substances effectively absorbing UV-A upon application to the skin have been known. Currently known substances are only dibenzoylmethane derivatives and cinnamic acid derivatives, most of which are liposoluble (West Germany Published Pat. Nos. 2728241 and 2728243, Japanese Laid-open Patent Application Nos. 51-61641, 52-46056 and 57-197209). Only a few are soluble in water (Japanese Laid-open Patent Application No. 57-59840).
If it is intended to add these UV-A absorbers to cosmetics, limitations are placed on the properties of a cosmetic substrate. Accordingly, there is a demand for a development of more widely usable UV-A absorbers. Such UV-A absorbers should satisfy the following requirements.
(1) A maximum absorption wavelength in the vicinity of 350 nm.
(2) A sufficiently high molar extinction coefficient (.epsilon.) at the above-indicated wavelength.
(3) A small absorbance in the visible light range because coloration as cosmetic compositions is not desirable, i.e. .epsilon.=0 over 400 nm.
(4) Good stability against heat and light.
(5) No toxic, irritative and other noxious actions on the skin.
(6) Good compatibility with cosmetic substrates.
(7) Difficulty in percutaneous absorption and in removal such as by perspiration upon application to the skin, with the effects of the absorber being effectively lasted.
(8) Inexpensiveness.
Moreover, there is also a demand of a development for UV absorbers which are effective not only for UV-A, but also for UV-B. Such UV-absorbers should satisfy, aside from the above requirements (1) through (8), the requirement that they have a maximum absorption in the vicinity of 290 nm.